Metal- and Ligand-Centered Reactivity of meta-Carboranyl-Backbone Pincer Complexes of Rhodium

Abstract

We report the synthesis of the chelating phosphinite-arm carboranyl POBOP-H (POBOP = 1,7-OP(i-Pr) 2 -m-carboranyl) ligand precursor, preparation of its rhodium complexes, and their reactivity in oxidative addition/reductive elimination reactions. The oxidative addition of iodobenzene to the low-valent (POBOP)Rh(PPh 3 ) resulted in the selective formation of the 16-electron complex (POBOP)Rh(Ph)(I), featuring a highly strained exohedral rhodium−boron bond. The complex (POBOP)-Rh(Ph)(I) is the first example of a B-carboranyl aryl metal complex, which is a proposed intermediate in metal-promoted B−C coupling reactions. The complex (POBOP)Rh(Ph)(I) was selectively and directly converted, in the presence of acetonitrile, to (POB(BPh)OP)Rh(H)(I)(CH 3 CN) (POB(BPh)OP = 1,7-OP(i-Pr) 2 -2-Ph-m-carboranyl) through unprecedented cascade reductive elimination of the phenyl-B-carboranyl and the oxidative addition of a vicinal B−H bond of the boron cluster to the metal center, exhibiting both metal-and cluster-centered reactivity.

Expansion of the (BB)Ru metallacycle with coinage metal cations: formation of B–M–Ru–B (M = Cu, Ag, Au) dimetalacyclodiboryls

“… 91 – 95 The Ru1–B1 bond is strained as indicated by the acute B2–B1–Ru1 angle of 78.5(1)°, which dramatically deviates from the analogous angle for the unstrained B–H bond in the ligand precursor POBOP-H (116.1(1)°). 96 This distortion is likely caused by attractive Ru1···Cu1 and Ru1···B2 interactions that bring the ruthenium metal center closer to the B2–Cu1 bond thus causing the Ru1–B1 bond strain.…”

Rhodium‐Catalyzed Regioselective Hydroxylation of Cage B−H Bonds of o‐Carboranes with O2 or Air

“…We accidently discovered a rhodium(III)‐catalyzed B4‐hydroxylation reaction by refluxing a toluene solution of 1‐COOH‐2‐CH 3 ‐ o ‐C 2 B 10 H 10 ( 1 a ) in the presence of 5 mol % [{Cp*RhCl 2 } 2 ] and 2 equivalents of KOAc open to the air. The significance of the present work is threefold: 1) O 2 or air is employed as a reagent and the sole oxidant, thus making this method very environmentally benign and practical; 2) to the best of our knowledge, this is the first example of rhodium‐catalyzed hydroxylation with molecular oxygen, although rhodium‐promoted cage B−H,, or organic C−H functionalization has been extensively investigated; and 3) this also represents the first example of catalytic cage B−H hydroxylation of o ‐carboranes.…”

Forging Unsupported Metal–Boryl Bonds with Icosahedral Carboranes

“…Compounds containing supported M−B bonds, in which the source of boron is carborane, can be prepared by a number of methods, but as we are interested in the derivatization of the unfunctionalized carborane, directed B−H activation strategies are not applicable. Formation of compounds featuring an unsupported M−B bond, in which the source of boron is carborane, are rare compared to their more common boryl counterparts (of the type M–BR 2 ), with crystallographically characterized examples in the literature limited to mercury‐, tin‐, and thallium‐containing compounds synthesized by direct electrophilic substitution at the boron vertex…”